For space vehicles venturing beyond Earth orbit into deep space, current methods require frequent interaction and communication with Earth stations, which can significantly increase mission scheduling and operational costs.
The NASA Deep Space Network (DSN) is the primary provider of navigation and communication for the U.S. and its partnering nations on deep-space missions. DSN's capability has achieved mission success throughout its over fifty years of operation. However, as exploration initiatives increase and operational usage expands, the DSN has the potential for over-subscription due to its many ongoing and future planned missions, and thus stands to benefit from supplemental navigation augmentation capabilities designed to reduce DSN operations cost.
In addition to improved operational support, expanded exploration of the Solar System beyond current day capabilities requires innovative, non-conventional techniques for vehicle navigation. Very few existing systems can provide this additional service while reducing DSN workload. Therefore, new methods are required that support the DSN system by alleviating any operational interruptions and providing for increased operational autonomy of deep space vehicles.
A previous study of a navigation system based on variable celestial X-ray sources (in the range 0.1-20 keV), referred to as the X-ray Navigation and Autonomous Position Verification program, or XNAV, has shown the capability to support DSN measurements for deep space missions. XNAV relies on pulsars located at known positions on the sky and a pulse-timing model of the expected arrival time of each pulse. The periodic nature of these pulsar sources provides a reliable signal that can be continually detected and tracked. An XNAV range measurement is calculated using an observed pulse profile on a spacecraft and the predicted pulse arrival time from each pulsar's model. The observation time required to produce each XNAV measurement depends on each pulsar's unique characteristics and the spacecraft's detector qualities. A shortcoming of the XNAV technique is that many X-ray pulsars are faint and require extended observation times to generate sufficient usable data.
It would be desirable to provide a system capable of addressing the challenges of future DSN operations and enhancing position accuracy for deep space vehicles, as well as extending the XNAV navigation concepts to celestial sources emitting much higher energy photons than the celestial X-ray sources.
Gamma-ray bursts (also referred to herein as GRBs) are the most powerful explosions known in the Universe. They are extremely luminous, with many orders of magnitude more energy output in a few seconds than the Sun emits in a year. GRBs are theorized to be produced during the evolutionary end-stages of single and binary star systems. This includes the unusually energetic supernova explosions (so-called hypernovae), the merger of two neutrons stars, or when a small star is consumed by a black hole.
GRBs have been detected approximately once per day by past and existing science missions, although they are theorized to occur at a much higher rate due to the concept of beaming, in which the emissions from a burst are focused into only 1/100th of the total sky.
Thousands of GRBs have been detected since they were initially discovered in 1967 by the Vela satellites. GRB events are typically named and catalogued according to their detection date, in the format GRBYYYYMMDDx, where x is an optional letter designation for cases in which multiple bursts occur on a given day. These sources are typically detectable via their emissions in the tens of keV to MeV, and often higher, photon energy bands.
GRBs are typically classified morphologically into a few distinct classes, based on temporal and flux characteristics. Using the Term T90 as the time over which the burst emits from 5% to 95% of its total photon counts, long bursts are those with T90>2 sec, and are thought to be related to massive star collapse. Short bursts exhibit duration of T90<2 sec.
Another classification approach is fluence, S, which is the photon flux integrated over time. High fluence bursts exhibit S>1.6×10−4 erg/cm2/T90, whereas low fluence bursts are those with S<1.6×10−4 erg/cm2/T90. Most bursts exhibit some degree of a fast-rise and exponential-decay, referred to as FRED, behavior.
Short bursts are known to have harder spectra than long bursts, where a greater proportion of the detected photons are of higher energy. The importance of spectral properties, coupled with the sensitive energy band, E, of a given detector, can be seen in the relative statistics of GRB detection between instruments and missions.
As an example, the mission Fermi spacecraft's Gamma-ray Burst Monitor (GBM), with an effective area a factor of ˜3 smaller than that of Swift's Burst Alert Telescope (BAT), detects 1.5 times more GRBs per year. The reason for this dramatic difference is, in part, GBM's greater sky coverage, but also that GBM's sensitivity extends over a much broader energy band (8 keV≦E≦30 MeV) than does BAT (15 keV≦E≦150 keV).
Because the GBM's higher-energy response is a better match to the hard-spectrum emission from short bursts, a significantly larger fraction of bursts detected by GBM are short, compared to BAT.
The gamma-ray emissions of nearby neutron stars are visible as their radiation beams are swept across the Earth's line of sight by the stars' rotations. Both young neutron stars—with spin periods of tens of milliseconds and magnetic field strengths of order 1012 G—and those that have been recycled in a past mass-accretion evolutionary episode (leaving them with spin periods less than 10 ms and 109 G magnetic fields) are visible as sources of pulsed gamma-rays. The latter exhibit highly predictable timing behavior, enabling applications that rely on the regularity of their pulsations.
Catalogues of rotation-powered pulsars from which pulsed γ-rays have been detected from the Fermi and INTEGRAL missions detail basic properties and γ-rays fluxes that drive exposure times required for useful navigation precision. The catalogue includes both rotation-powered pulsars and soft-gamma repeaters (SGRs), which are bright, flaring, recurring sources.
The Fermi team has reported, in its published Second Source Catalog, 83 rotation-powered pulsars from which pulsed γ-rays have been detected. Among the pulsed sources, fluxes are typically at the level of 10−8 photons/cm2/sec. The brightest, the Vela pulsar, has a flux of 3.4×10−6 photons/cm2/sec. These fluxes are integrated over the energy band 0.3 MeV to 1 GeV, where the (hard) γ-ray emissions of rotation-powered pulsars are brightest—below a hundred MeV. Galactic background emission reduces signal-to-noise ratios considerably. Above a few GeV, pulsar spectra cut off exponentially.
The Fermi Large Area Telescope (LAT) effective collecting area in the 0.3-1 GeV band is approximately 6,000 cm2, so that for the Vela pulsar, the detected photon flux is 0.02 counts/sec. While this flux level is potentially conducive to navigation analysis in the manner of XNAV, scaling the large sized LAT down to a detector size that would be appropriate to a navigation subsystem would significantly reduce the photon detection rate. Additionally, for a more-typical fainter γ-ray pulsar, the photon flux is two orders of magnitude lower.
In the energy band at the low end of GRB emissions (tens to 100 keV), the INTEGRAL satellite provides a good measure of typical fluxes for both rotation-powered pulsars and so-called magnetars (SGRs and anomalous X-ray pulsars that are believed to be powered by the slow decay of their enormously strong magnetic field). In its survey mode, INTEGRAL detected three pulsars, two AXPs (Anomalous X-ray pulsars), and two SGRs (Soft Gamma Repeaters). These pulsars are among the brightest in their classes (pointed, non-survey INTEGRAL observations are much more sensitive to dimmer sources). Typical fluxes in both the soft (20-40 keV) and hard (40-100 keV) INTEGRAL bands for these seven objects are in the vicinity of 3×10−4 ph/cm2/sec. With INTEGRAL's effective area of 2,600 cm2, a detected photon flux of ˜1 count/sec is produced.
These detected INTEGRAL fluxes are for sources in their quiescent state. Rotation-powered pulsars are not variable in flux, but both magnetar varieties exhibit sporadic, unpredictable flares; those from SGRs can be exceedingly bright. These SGR flares are believed to recur every few years. AXP flares, on the other hand, increase the quiescent flux by a factor of a 2-5, and recur every few days-to-weeks, with larger flares being less frequent.
These low photon flux rates make the use of γ-ray pulsars an extreme challenge for a practical navigation system. Thus, the more useful γ-ray sources are those of the high flux GRB type.
It is therefore desirable to provide a new technique for determining spacecraft navigation solutions using high-energy events photons from distant celestial gamma-ray burst (GRB) sources.